Iphone 6s - Email notifications disappear from Notification centre when unlocking phone

Hey guys,

I have a problem with Apple's Stock Mail App or maybe with the notifications at all.

When I get an Email (it happens with all of my mail accounts) and the phone lies locked on my desk I get a notification on the lock screen (everything isnormal here), I unlock the phone, do something else and lock the phone again - the notification from the lockscreen is gone as well (also normal so far) BUT when I swipe do the notification centre, the mail notification is gone as well ALTHOUGH I haven't read the Mail and did not dismiss the notification. When the phone is locked, I get an email notification, I swipe down to notification centre while the phone is still locked the notification is there, once I press my finger on the touch ID and unlock my phone the Email notificaion dismisses itself from the notification centre.
I haven't figured out why... This problem occured when updating form 10.1.1 to 10.2

All other App notifications from example whatsapp, facebook, DHL all stay on the notification centre until I dismiss them or press them - 3rd party Mail apps work perfectly with notificaions!
Additional steps I took:

- Resetting device multiple times

- Resetting mailaccount

- Trying to figure out if a mailaccount causes problems

- Using a third party mail app where notifications stay like other apps
- Checking notification settings on the mail app and on the iphone

And other stuff I don't remember yet. I had a 1 hour call with Apple today and we resetted the phone etc. it worked for a short time and then the problem occured again and I don't have a clue why.
Maybe someone of you experienced the same?

Thanks for your help!